I'm building a sub based on two Rythmik Audio servo kits, installed into a dual chamber sealed box. The kits came with both a rubber gasket that completely surrounds the rim of the driver (see below):

as well as a thin gasket of foam rubber that can alternatively be applied to the box before mounting the driver. Since the foam rubber gasket was damaged in the box (it seems the weight of the driver ended up on a small portion of the gasket, causing it to be permanently compressed), I chose to use the much thicker rubber gasket.

Although I haven't yet drilled the mounting holes in my box, I did a quick test of the one of the driver/amp pairs, by placing it in the hole and simply holding down in place. However, no matter how much pressure I applied to the rim, I could still hear air leaking around the gasket at low frequencies. I realize I'm not going to be able to apply as much pressure as screwing the driver into place, but I'm concerned that it may still not provide a complete seal.

Does anyone ever use any type of caulking between the rubber gasket and the box? I'd prefer to find something that would help provide a better seal, but not have any adhesive properties that would prevent the driver from being easily removable.

I have an Ascendant Audio Atlas subwoofer that has an almost identical rubber "gasket" around the edge. I didn't put anything on it to seal it and there are no leaks whatsoever. The rubber works quite well. For simplicity, would it not be easier to finish the sub using nothing, then if there are leaks you can take the driver out and add a seal.

Just to follow up, I found a good product for this situation. I wanted something the consistency of Vaseline, but wanted to make sure that it wouldn't compromise my surrounds if any inadvertantly got on them. It seems the surround of my TC2+ drivers are rubber, specifically NBR. Based on the research I did, petroleum jelly is relatively safe for NBR (unlike latex rubber, used in condoms, which is severely weakened by petroleum products), though silicone is preferable (and even protects and extends the life of rubber). There is a very thick product called "plumber's grease" that is used to lubricate, protect, and seal o-rings and other components in faucets, etc. From what I've read, there's both petroleum based and silicone based plumber's grease. I found a small tube of it at Ace, though there was nothing to give any indication of what it's base was. After I got it, it SEEMED to me like it was petroleum based (silicone has a different feel). Since it's used in conjunction with rubber washers and o-rings, it should be fairly safe, but I still preferred silicone. So I found a small tub of "silicone plumbers grease" at Lowes.

Smeared it on the gasket where it comes in contact with my box, as well as the rim of the driver that inserts into the gasket. On one of my two drivers, it worked perfectly. The other one was fine around the gasket, yet still had some slight air noise at some of the screw holes at low frequency high excursion signals. Since the petroleum based product came in a tube and was therefore easier to squirt into a hole, I used it: removed the first screw where I could tell was leaking some air, put some of it into that hole, then screwed the screw back in. Then hunted down the next leak. After three screws, it was air tight! I have absolutely no sound at all during high excursion infrasonic tones.

If you use a foam of any considerable thickness, I'd recommend
it be low density, such as weather stripping foam. High density foam
if compressed too far under the screw points can exert too much stress on the frame at the points between the mounting screws. Could warp a stamped frame or crack a cast one.